Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of relatively low-voltage DC power. Because power is generally delivered through a wall outlet as high-voltage AC power, a device, typically referred to as a power supply or power converter is required to transform the high-voltage AC power to low-voltage DC power. The low-voltage DC power may be provided by the power supply directly to the device or it may be used to charge a rechargeable battery that, in turn, provides energy to the device, but which requires charging once stored energy is drained. Typically, the battery is charged with a battery charger that includes a power supply that meets constant current and constant voltage requirements required by the battery. In operation, a power supply may use a controller to regulate output power delivered to an electrical device, such as a battery, that may be generally referred to as a load. More specifically, the controller may be coupled to a sensor that provides feedback information of the output of the power supply in order to regulate power delivered to the load. The controller regulates power to the load by controlling a power switch to turn on and off in response to the feedback information from the sensor to transfer energy pulses to the output from a source of input power such as a power line.
One of the items often included in power supplies is a transformer. A voltage is usually applied to the transformer from one part of the power supply circuit (often referred to as the primary side) and the transformer transfers energy from that part of the power supply circuitry to another part of the power supply circuitry (often referred to as the secondary side). The transformer provides the galvanic isolation and also shifts the voltage level between the primary side and secondary side of the power supply. More specifically, galvanic isolation occurs when DC current is unable to flow between the primary and secondary sides of the power supply.
The design and construction of power supplies as well as the transformers that are used in the power supplies can be a very complicated and complex task due to many factors and variables. The complexity is further increased if the power supply is to include multiple outputs. For example, the design and construction of power supplies is often done through experimentation, on a trial and error basis in order for the power supply designer to realize a power supply design having the desired specifications with regard to electrical performance and cost.